The present invention relates to an electrophotographic image forming apparatus, and a cleaning roller for cleaning a charging roller used in this image forming apparatus.
An electrophotographic image forming apparatus has an electrostatic latent image bearing member which carries an electrostatic latent image. In operation for forming an image by the apparatus, an outer peripheral surface of the electrostatic latent image bearing member is uniformly charged to a certain potential by a charging device, and thereafter an electrostatic latent image is formed on the electrostatic latent image bearing member by exposure using an exposure device. The electrostatic latent image formed on the electrostatic latent image bearing member is developed by toner supplied from a developing device, thereby forming a toner image on the electrostatic latent image bearing member. The toner image formed on the electrostatic latent image bearing member is transferred to a recording sheet such as a paper sheet, directly or through an intermediate transfer member. The recording sheet with the toner image transferred thereto is heated or pressurized by a fusing device so that the toner image is fixed on the recording sheet.
Conventionally, a roller type charger has been used for the charging device. The charging roller is disposed in contact with the outer peripheral surface of the electrostatic latent image bearing member. By an application of a charging voltage to the charging roller, the outer peripheral surface of the electrostatic latent image bearing member is charged to a certain potential. Typically, the charging roller has a metal core and a conductive elastic layer covering an outer periphery of the metal core. A solid rubber such as a resin foaming member is used for the conductive elastic layer, and conductivity is provided to the charging roller by the addition of carbon black or metallic powder.
In this apparatus, foreign matters such as external additives of toner or the toner particles on the electrostatic latent image bearing member can be transferred and attached to the outer peripheral surface of the charging roller. The foreign matters attached to the charge roller can cause a charge defect, which results in a deterioration of the resultant images.
In order to solve the aforementioned problem, there is proposed a technique in which the cleaning roller for removing the foreign matters attached to the outer peripheral surface of the charging roller is disposed in contact with the outer peripheral surface of the charging roller.
Japanese Patent Publications JP 2006-330613 A, JP 2004-361916 A, JP 2005-227411 A disclose a technique of using a cleaning roller for cleaning the charging roller.
The cleaning roller disclosed in JP 2006-330613 A is characterized in that the surface of the cleaning roller for cleaning the charging roller is made of a polyurethane foam layer in which the number of cells of the polyurethane foam is increased. According to the cleaning roller, the cell wall surfaces of the polyurethane foam layer are brought into contacts with the charging roller to improve a scraping performance of the roller.
The cleaning roller disclosed in JP 2004-361916 A and JP 2005-227411 A is characterized in that the surface of the cleaning roller for cleaning the charging roller is made of a melamine-based resin layer with open-cell structure in which a density of the melamine-based resin layer is lessened. The melamine-based resin layer has an excellent flexibility so that foreign matters are readily scraped into the melamine-based resin layer. This causes the foreign matters to be accumulated on the cell surfaces of the melamine-based resin layer. Therefore, the charging roller is hardly damaged by the cleaning roller or the foreign matters on the surface of the cleaning roller.
However, in the cleaning roller disclosed in JP 2006-330613 A, each cell of the polyurethane foam layer is extremely small, and therefore the cells of the surface are easily occupied by the foreign matters such as an external additive. Therefore, the wall surface of the cells of the polyurethane foam layer is hardly brought into contact with the outer peripheral surface of the charging roller, and the scraping performance of the foreign matters is not necessarily sufficiently improved. In addition, when the polyurethane foam layer has a closed cell structure, the external additives are easily clogged or fixed to the cell wall surface inside of the cell surface. Therefore, the outer peripheral surface of the charging roller is easily damaged by the clogged and fixed matters of the external additives.
In the cleaning rollers disclosed in JP 2004-361916 A and JP 2005-227411 A, each cell of the melamine-based resin layer is rather large, and therefore the frequency of bringing the cell wall surface of the melamine-based resin layer into contact with the outer peripheral surface of the charging roller is lessened, thus deteriorating the scraping performance of the foreign matters. Further, since the melamine-based resin layer has the open-cell structure, the foreign matters scraped from the cell wall surface are easily dropped out of the melamine-based resin layer through another cell. In addition, since the foreign matters easily enter into the melamine-based resin layer, the foreign matters accumulated inside of the melamine-based resin layer are clogged by long-term use, thus deteriorating the collecting performance of the foreign matters.
Therefore, it is desired to provide the cleaning roller for cleaning the charging roller capable of efficiently recovering the foreign matters such as the external additive of the toner attached to the outer peripheral surface of the charging roller while preventing damage on the outer peripheral surface of the charging roller, and capable of excellently maintaining its recovery performance for a long period, and also it is desired to provide the image forming apparatus including this cleaning roller for cleaning the charging roller.